**Core Concept**
Live vaccines are made from weakened or attenuated forms of microorganisms, which stimulate an immune response without causing the full-blown disease. These vaccines can be administered alone or in combination with other vaccines to provide broad-spectrum protection.

**Why the Correct Answer is Right**
The statement that "Two live vaccines cannot be administered simultaneously" is true because co-administration of multiple live vaccines can lead to an increased risk of adverse reactions, such as vaccine-associated illnesses or exacerbation of underlying medical conditions. This is because live vaccines can shed the attenuated microorganisms, potentially causing infection in immunocompromised individuals or individuals with certain underlying conditions. The Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) provide guidelines on the safe administration of live vaccines in combination.

**Why Each Wrong Option is Incorrect**
**Option B:** This statement is incorrect because booster doses are often required for live vaccines, especially for certain diseases such as measles, mumps, and rubella (MMR). Booster doses help maintain immunity levels and prevent waning of antibody titers over time.

**Option C:** This statement is incorrect because single doses of live vaccines do not always provide lifelong immunity. For example, live attenuated influenza vaccine (LAIV) requires annual administration to maintain protection against circulating influenza strains. Additionally, some live vaccines, such as the MMR vaccine, require two doses to ensure adequate immunity.

**Option D:** This statement is incorrect because live vaccines typically contain only major antigens, which are the primary components of the microorganism that stimulate an immune response. Minor antigens, which are secondary components, are not typically included in live vaccines.

**Clinical Pearl / High-Yield Fact**
It's essential to follow established guidelines for the administration of live vaccines to minimize the risk of adverse reactions and ensure optimal immunity. Always check the CDC and WHO guidelines for the most up-to-date information on live vaccine administration.

**✓ Correct Answer: A. Two live vaccines cannot be administered simultaneously.**